(1) Field of the Invention
The present invention relates generally to bulk material feeders. More particularly, the invention is a precision gravimetric feeder for precisely dispensing solid particulate materials.
(2) Description of the Prior Art
Numerous solid particulate materials are packaged in receptacles for sale to the ultimate consumer or downstream processor. A brief exemplary listing of such materials include plastic and metal components and parts; food items, such as cereals, corn meal, rice, spices, soybeans, and a variety of other materials, such as tobacco, plastic pellets, etc. Exemplary receptacles include boxes, containers, pouches, packages, cartons, and bags.
Packages of these products may be labeled according to the number of items contained therein or by the weight of the contents. In practice, these products are packaged by a filling system that places a predetermined volume of the solid particulate material product into package containers. The filled packages are then weighed and subsequent packages are adjusted in volume until the proper weight per package is achieved.
This method of dispensing solid particulate material products is problematic in that it results in packages that are significantly under or over weight. Accordingly, the contents of these packages must be discarded or recycled back into the filling system. The problem is caused by the contents of the packages being placed into the package container before the contents are weighed, thus making it impossible to control particular package weights. What is needed is an apparatus that precisely measures the desired weight of solid particulate material as the material is being packaged.
It has been found that this need can be addressed by dispensing a measured initial weight of particulate material comprising most of the desired weight, plus a measured fill weight of particulate material equal to the difference in the desired weight and the initial weight. A precise fill weight is achieved by discharging the material through a small area opening to limit the quantity of material dispensed during a given time, with a dispensing valve shutting off the flow of fill material when the desired fill weight has passed through the opening.
More specifically, particulate material supplied from a material source such as a hopper is initially captured in a trap chamber positioned below the hopper. The trap chamber has a chamber inlet in communication with the hopper and an outlet in the lower part of the chamber to discharge material from the trap chamber. The chamber inlet is preferably above the chamber outlet, so that particulate material exiting the chamber inlet falls downward through the chamber outlet. Importantly, the area of the chamber inlet is smaller than the chamber outlet, e.g., the area of the inlet is from about 25% to about 75% of the area of the outlet.
An actuator-controlled valve having open and closed positions covers the chamber outlet. A flow meter is positioned to receive and weigh the initial material weight and fill material exiting the trap chamber. When the total weight of material measured reaches a predetermined value, the flow meter instructs the actuator to move the valve to its closed position. The total weight of material measured will be slightly less than the weight of material dispensed, since a small quantity of material will be between the valve and the flow meter at the time the signal is generated. Thus, the measured weight will be equal to the weight actually measured, plus this in-transit weight. Due to the relatively small size of the chamber inlet, the weight of material passing the valve at the time the signal is received is relatively small. Therefore, a high degree of accuracy is possible.
Operation of the present invention begins with the dispensing valve closed. As soon as an initial weight of material collects in the trap chamber, the dispensing valve can be rapidly opened, releasing the initial weight of material to engage the flow meter. The flow meter totals the initial weight of particulate material together with a trickle of particulate material that follows the release of the initial weight. As the particulate material total approaches the desired weight, the flow meter generates a signal that commands the valve actuator to close the dispensing valve, shutting off the trickle of particulate material flowing from the trap chamber outlet. The dispensed weight may be slightly less than the desired weight depending on the geometry of the trap chamber and the speed of the dispensing valve closure.
In particular, the trap chamber is provided to collect an initial weight of particulate material that is less than the desired weight of particulate material. Preferably, the trap chamber is sized to hold an initial weight of material that is less than the desired weight from about 50% to about 90%.
The dispensing valve has a closed position covering the trap chamber outlet and an open position uncovering the trap chamber outlet. Furthermore, a valve actuator is in communication with the valve to open and close the trap chamber outlet. The actuator can be, but is not limited to electromechanical, pneumatic or hydraulic types. The actuator responds to a signal generated by the flow meter. The signal can be an analog signal or a logic signal. In either case, the signal will command the actuator to close the trap chamber valve.
The preferred dispensing valve is made up of a housing having an aperture and a gate member that is adapted for sliding movement through the aperture between a valve-open position and a valve-closed position. The aperture of the gate valve is at least as large as the trap chamber outlet. In the valve-closed position, the gate member extends across the trap chamber outlet to close off the flow of particulate material. Preferably, the valve actuator can slide the gate member from the valve open position to the valve-closed position within at least about 50 milliseconds. Similarly, it is preferred that the actuator slides the gate member from the valve-closed position to the valve open position within at least about 50 milliseconds.
The flow meter can be a solid particle mass flow meter used as a dynamic weighing apparatus for accurately weighing particulate material while the particulate material is in motion. Preferably, the solid mass flow meter used as a part of the present invention can accurately weigh solid mass flow rates between the range of 5 lb/min. and 5000 lb/min. Particularly suitable solid particle mass flow meters are described in earlier U.S. Pat. No. 5,219,031, issued Jun. 15, 1993 and U.S. Pat. No. 5,230,251, issued Jul. 27, 1993, to the present inventor.
Generally, the solid particle mass flow meter is comprised of a curved weigh pan having inlet and outlet ends, and an inwardly curved surface extending between pan ends. The pan is positioned to receive a continuous stream of solid particulate material tangentially at the inlet end, i.e., the material stream is directed substantially perpendicular to the radius of curvature of the pan at the upper end. As a result, the material flows around the curved surface of the pan without impacting the pan, eliminating errors due to the movement of the pan under impact forces.
The pan is mounted on the distal end of an elongated support arm, with the proximal end of the arm being attached to a suitable support at a distance from the pan. To permit pan displacement, the arm is either flexible or is pivotally attaching at its proximal end. When material flows over the inwardly curved pan, an inward centripetal force, and corresponding outward force, is exerted causing the pan to move outwardly. This outward movement, corresponding to the weight, or change in weight, of material moving across the pan is measured by a displacement measurement instrument such as a transducer, which is positioned to measure the displacement of the pan.
Since the outward force exerted against the pan is due entirely to the centripetal force, and is independent of any impact or frictional forces, measurement is highly accurate and can be measured instantaneously. For optimal measurement independent of non-centripetal force factors, the pivot point of the arm is preferably located so that a line extending from the arm to the pan approximately midway between the inlet and outlet ends is perpendicular to the radius of curvature of the pan.
Operation of the invention typically begins with the dispensing valve in the valve-closed position. As material is loaded into the hopper, an initial weight of material collects in the trap chamber as particulate material dribbles through the trap chamber inlet. Once the trap chamber is adequately filled, precise dispensing of the particulate material may begin.
The dispensing process starts upon receipt of a signal that indicates an empty container is ready to be filled. The actuator responds to the signal by moving the valve to its open position, releasing the initial weight of particulate material. The initial weight of solid particulate material falls through the outlet of the trap chamber and is directed onto the solid mass flow meter""s weigh pan tangential to the pan inlet end. The material moving across the pan surface is discharged from the pan outlet end preferably into a packaging receptacle. As soon as the initial weight is released, additional particulate material dribbles through the trap chamber inlet and into the trap chamber. However, the dispensing valve remains in the valve-open position and the trap chamber does not collect the dribble of particulate material. Instead, the dribble of material flows freely through the trap chamber outlet and, like the released initial weight, is directed onto the solid mass flow meter""s weigh pan tangential to the pan inlet end. As the dribble of particulate material continues to flow, the solid mass flow meter measures the difference between the released weight and the desired weight. When the total measured weight of particulate material reaches a predetermined value, the solid mass flow meter generates a signal that commands the dispensing valve to abruptly close, thus gravimetrically dispensing a precise weight. The desired weight equals the predetermined value plus an in-transit amount of particulate material. The present invention is capable of dispensing weights that are within 0.1% of the desired weight. Due to the small surface area of the trap chamber relative to the trap chamber outlet, the flow rate of particle material exiting the trap chamber outlet after the release of the initial weight is significantly less than the average flow rate. As a result, errors involving valve closure speed and repeatability are greatly reduced. For example, a 75% reduction in flow rate after the release of the initial weight, results in a dispensing accuracy increase of ten-fold.